The present invention relates to a method for pre-treatment of crude water in membrane separation, especially for that in reverse osmosis for desalination or separation, for example, in reverse osmosis for desalination of seawater, to a method of bacteriostasis or disinfection for membranes, and to an apparatus for them.
Membrane separation is much used in various fields of desalination of seawater and saltwater, production of pure water and ultrapure water for medical and industrial use, treatment of industrial drainage, etc. In such membrane separation, contamination of the membrane separation apparatus with microorganisms worsens the quality of permeates and lowers the membrane permeability and separability owing to the growth of microorganisms on and around the membranes or to the adhesion of microorganisms and their metabolites onto them. Concretely, the influences of microorganisms result in the degradation of the quality of permeates, the reduction in the amount of permeates, the increase in running pressure or in the increase in pressure loss. In order to evade such serious problems, various techniques and methods for bacteriostasis and even disinfecting microbes in membrane separation units have heretofore been proposed. For example, microbicides are used. Most generally, a chlorine-containing microbicide, of which the effect has been verified and which has the advantages of low cost and easy handlability, is added to membrane separation units at a concentration of from 0.1 to 50 ppm or so. One general method of using such a microbicide comprises adding a microbicide to a pre-treatment zone in a membrane separation apparatus, in which the pre-treated water having been subjected to disinfection with sodium hypochlorite and then to flocculation and filtration is, before being fed into the membrane separation units, once stored in a tank, and then processed for removing free chlorine from it through reduction with sodium bisulfite before the safety filter as disposed in the zone before the membrane treatment units.
Chlorine-containing microbicides chemically degrade reverse osmosis membranes. Therefore, when they are used, free chlorine from them must be reduced with a reducing agent before they reach reverse osmosis membranes. As the reducing agent, generally used is sodium bisulfite in an amount of from 1 to 10-fold equivalents. The concentration of the reducing agent is determined in consideration of its ability to completely remove the remaining microbicide and of the probability of its reacting with dissolved oxygen in the system being treated. However, even when a membrane separation apparatus is run in a continuous running manner according to that method of using such a chlorine-containing microbicide, its membrane capabilities are often still worsened, and it has been found that the method is not always satisfactory for disinfecting microorganisms in the apparatus. In this connection, it is said that the chlorine as added in the method oxidizes the organic carbons existing in the crude water being treated, whereby the thus-oxidized organic carbons are converted into compounds that are readily decomposed by microorganisms (see A. B. Hamida and I. Moch, Jr., Desalination and Water Reuse, 6/3, 40-45, 1996), but their theory has not been verified as yet. Given that situation, another method for membrane disinfection has been developed, which comprises intermittently adding sodium bisulfite to a membrane separation system generally at a concentration of 500 ppm. This method has become used in practice, but, in some cases, it is not still effective. Those having tried the method have often experienced deposition of microorganisms on permselective membranes.
In the conventional pre-treatment method, the pre-treated water having been subjected to disinfection and to flocculation and filtration is stored in a tank for a while, which, therefore, is often contaminated with some external contaminants whereby microorganisms much grow in the thus-contaminated, stagnant water to further worsen the quality of the water. The disinfecting effect of sodium bisulfite to be used in the method is for removing oxygen from the crude water being processed and to lower the pH value of the crude water. However, while a permselective membrane apparatus is run according to the method, the intermittent addition of sodium bisulfite to the apparatus is not all the time effective for disinfecting the membrane in the apparatus. We, the present inventors have studied the reason, and have found that ordinary aerobic bacteria that grow in a neutral or alkaline condition could be prevented from growing in an anaerobic environment in some degree but could not be killed in that environment. Having noted it, we have reached the conclusion that the pH depression in the system where bacteria may live is rather the most effective for disinfecting the bacteria therein. That our conclusion is not contradictory to the microbiological viewpoint in this respect. On the other hand, we have further found that, even when a high concentration of sodium bisulfite of 500 ppm is added to crude water having a high salt concentration such as seawater, the pH value of the water system could not be lowered to such a degree that ordinary bacteria existing therein could be killed. Therefore, it is understood that sodium bisulfite added to crude water having a lower salt concentration could exhibit its disinfecting effect not in an anaerobic condition but rather in a lowered pH condition. Accordingly, we have found that adding a high concentration of expensive sodium bisulfite to membrane separation units is not needed for disinfecting them but merely adding inexpensive sulfuric acid or the like thereto to lower the pH value in the system around them is satisfactory for disinfecting the units, and that, when the pre-treated water is prevented from standing for a while in a tank or the like in a water treatment apparatus, then the growth of microorganisms in the apparatus could be inhibited. On the basis of these findings, we have completed the present invention.
The object of the invention can be attained by the constitution mentioned below. Specifically, the invention provides xe2x80x9ca method of bacteriostasis or disinfection for a permselective membrane in a membrane separation apparatus for water purification, which comprises a step of treating crude water with an acid at a pH of at most 4xe2x80x9d, and also provides a method for purifying water that essentially comprises the disinfection method, and an apparatus for the method.